Neurotransmitter release from many central nervous system synapses is regulated by 'autoreceptors' at the synaptic terminal, which bind the released transmitter and alter release accordingly. The photoreceptors of lower vertebrates are thought to use glutamate as a neurotransmitter. Glutamate conveys the visual signal to postsynaptic bipolar and horizontal cells, but has been reported not to act on the photoreceptors themselves. We show here that glutamate evokes a current, carried largely by chloride ions, in cones isolated from the tiger salamander retina. This response is localized to the synaptic terminal of the cone. Removing external sodium blocks this action of glutamate. These results suggest the existence of a positive feedback loop at the cone output synapse: over most of the light-response range, glutamate released by depolarization of the cone will cause further depolarization, increasing the gain of phototransduction. Glutamate released from rods may also polarize cones, modulating the gain of the cone output synapse. This system is surprisingly different from the autoreceptor systems for most other transmitters, which act in a negative feedback way.